Method and apparatus for transmission and reception of compressed audio frames with prioritized messages for digital audio broadcasting

ABSTRACT

A method for transmission of compressed data for a digital audio broadcasting system comprises the steps of producing digital information representative of an audio signal; estimating the number of bits to be allocated to the digital information in a modem frame; encoding the digital information within the estimated number of bits to produce encoded data; removing selected bits from the encoded data; adding bits corresponding to digital messages to the encoded information to form a composite modem frame; formatting the composite modem frame bits to produce formatted composite modem frame bits; and transmitting the formatted composite modem frame bits. The invention also encompasses transmitters that perform the method.

BACKGROUND OF THE INVENTION

This invention relates to methods and apparatus for transmitting andreceiving digital data, and more particularly, to such methods andapparatus for use in digital audio broadcasting systems.

Digital Audio Broadcasting (DAB) is a medium for providingdigital-quality audio, superior to existing analog broadcasting formats.Both AM and FM DAB signals can be transmitted in a hybrid format wherethe digitally modulated signal coexists with the currently broadcastanalog AM or FM signal, or in an all-digital format without an analogsignal. In-band-on-channel (IBOC) DAB systems require no new spectralallocations because each DAB signal is simultaneously transmitted withinthe same spectral mask of an existing AM or FM channel allocation. IBOCDAB promotes economy of spectrum while enabling broadcasters to supplydigital quality audio to their present base of listeners. Several IBOCDAB approaches have been suggested. One such approach, set forth in U.S.Pat. No. 5,588,022, presents a method for simultaneously broadcastinganalog and digital signals in a standard AM broadcasting channel. Usingthis approach, an amplitude-modulated radio frequency signal having afirst frequency spectrum is broadcast. The amplitude-modulated radiofrequency signal includes a first carrier modulated by an analog programsignal. Simultaneously, a plurality of digitally-modulated carriersignals are broadcast within a bandwidth that encompasses the firstfrequency spectrum. Each digitally-modulated carrier signal is modulatedby a portion of a digital program signal. A first group of thedigitally-modulated carrier signals lies within the first frequencyspectrum and is modulated in quadrature with the first carrier signal.Second and third groups of the digitally-modulated carrier signals lieoutside of the first frequency spectrum and are modulated both in-phaseand in-quadrature with the first carrier signal. Multiple carriers areemployed by means of orthogonal frequency division multiplexing (OFDM)to bear the communicated information.

FM IBOC DAB broadcasting systems have been the subject of several UnitedStates patents including U.S. Pat. Nos. 5,465,396; 5,315,583; 5,278,844and 5,278,826. One hybrid FM IBOC DAB signal combines an analogmodulated carrier with a plurality of orthogonal frequency divisionmultiplexed (OFDM) sub-carriers placed in the region from about 129 kHzto about 199 kHz away from the FM center frequency, both above and belowthe spectrum occupied by an analog modulated host FM carrier. Anall-digital IBOC DAB system eliminates the analog modulated host signalwhile retaining the above sub-carriers and adding additionalsub-carriers in the regions from about 100 kHz to about 129 kHz from theFM center frequency. These additional sub-carriers can transmit a backupsignal that can be used to produce an output at the receivers in theevent of a loss of the main, or core, signal.

One feature of digital transmission systems is the inherent ability tosimultaneously transmit both digitized audio and data. Digital audioinformation is often compressed for transmission over a bandlimitedchannel. For example, it is possible to compress the digital sourceinformation from a stereo compact disk (CD) at approximately 1.5 Mbpsdown to 96 kbps while maintaining the virtual-CD sound quality for FMIBOC DAB. Further compression down to 48 kbps and below can still offergood stereo audio quality, which is useful for the AM DAB system or alow-latency backup and tuning channel for the FM DAB system. Effectivecompression schemes employ variable rate source encoding where fixedtime segments of audio are encoded into digital packets of variablelength, i.e. audio segments of varying “complexity” are converted intoaudio frames of varying length.

Audio frames generated by typical audio encoders are in formats that arenot efficient for transmission as an IBOC DAB signal. There is a needfor an efficient method for transmission and reception of compressedaudio frames for digital audio broadcasting.

SUMMARY OF THE INVENTION

A method for transmission of compressed data for a digital audiobroadcasting system comprises the steps of receiving digital informationrepresentative of an audio signal; estimating the number of bits to beallocated to the digital information in a modem frame; encoding thedigital information within the estimated number of bits to produceencoded data; adding bits corresponding to digital messages to theencoded information to form a composite modem frame; formatting thecomposite modem frame bits to produce formatted composite modem framebits; and transmitting the formatted composite modem frame bits.

The invention also encompasses modem frame formats produced by themethod and transmitters that perform the method. The modem frame formatsinclude a plurality of backup core audio fields, an enhanced audio/datafield, and a header field. Each of the backup core audio fields includesa core audio frame, a cyclic redundancy check bit, a redundant headerfield, and flush bits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a transmitter for use in a digital audiobroadcasting system that can transit signals formatted in accordancewith this invention;

FIG. 2 is a functional block diagram illustrating the method ofmultiplexing and encoding audio and prioritized data packets inaccordance with this invention;

FIG. 3 is a block diagram of a receiver that can process signals inaccordance with this invention;

FIG. 4 is a block diagram illustrating a portion of the signalprocessing performed in the receiver of FIG. 3;

FIG. 5 is a schematic representation showing a preferred embodiment ofthe modem frame format used with the present invention;

FIG. 6 is a schematic representation showing a preferred embodiment ofthe backup audio/supplemental frame format used with the presentinvention;

FIG. 7 is a schematic representation showing a preferred embodiment ofthe backup core audio frame of the modem frame format used with thepresent invention;

FIG. 8 is a schematic representation showing a preferred embodiment ofthe enhanced audio/data field of the modem frame format used with thepresent invention;

FIG. 9 is a schematic representation showing a preferred embodiment ofthe redundant header field of the modem frame format used with thepresent invention;

FIG. 10 is a schematic representation showing a preferred embodiment ofthe core modem frame format used with the present invention for use inan AM DAB system;

FIG. 11 is a schematic representation showing a preferred embodiment ofthe core audio block frame format used with the present invention foruse in an AM DAB system;

FIG. 12 is a schematic representation showing a preferred embodiment ofthe enhanced modem frame format used with the present invention for usein an AM DAB system;

FIG. 13 is a block diagram of the data signal interfaces that may beused when practicing this invention in a receiver for use in a digitalaudio broadcasting system; and

FIG. 14 is a block diagram of a data signal interface that may be usedwhen practicing the invention in a transmitter in a digital audiobroadcasting system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, FIG. 1, is a block diagram of a DABtransmitter 10 which can broadcast digital audio broadcasting signals inaccordance with the present invention. A signal source 12 provides thesignal to be transmitted. The source signal may take many forms, forexample, an analog program signal that may represent voice or musicand/or a digital information signal that may represent message data suchas traffic information. A digital signal processor (DSP) based modulator14 processes the source signal in accordance with various known signalprocessing techniques, such as source coding, interleaving and forwarderror correction, to produce in-phase and quadrature components of acomplex base band signal on lines 16 and 18. The signal components areshifted up in frequency, filtered and interpolated to a higher samplingrate in up-converter block 20. This produces digital samples at a ratef_(s), on intermediate frequency signal f_(if) on line 22.Digital-to-analog converter 24 converts the signal to an analog signalon line 26. An intermediate frequency filter 28 rejects aliasfrequencies to produce the intermediate frequency signal f_(if) on line30. A local oscillator 32 produces a signal f_(lo) on line 34, which ismixed with the intermediate frequency signal on line 30 by mixer 36 toproduce sum and difference signals on line 38. The sum signal and otherunwanted intermodulation components and noise are rejected by imagereject filter 40 to produce the modulated carrier signal f_(c) on line42. A high power amplifier 44 then sends this signal to an antenna 46.

The method of this invention involves the efficient and robustmultiplexing of compressed digital audio along with data messages ofvarying priority, or time urgency, requirements. A basic unit oftransmission of the DAB signal is the modem frame, which is on the orderof a second in duration. This duration is required to enablesufficiently long interleaving times to mitigate the effects of fadingand short outages or noise bursts such as may be expected in a digitalaudio broadcasting system. The delay for the main digital interleavedaudio channel can be no less than the duration of the modem frame.However, this delay is not a significant disadvantage since one IBOC DABsystem in which the invention may be used already employs a diversitydelay technique, which intentionally delays the digital signal forseveral seconds with respect to the analog signal. A DAB system whichincludes time diversity is described in commonly owned U.S. patentapplication Ser. No. 08/947,902, filed Oct. 8, 1997, now U.S. Pat. No.6,178,317. All analog or digital time diversity signal is provided forfast tuning acquisition of the signal. Therefore the main digital audiosignal is processed in units of modem frames, and any audio processing,error mitigation, and encoding strategies should be able to exploit thisrelatively large modem frame time without additional penalty.

In this invention, a format converter is used to repackage thecompressed audio frames in a manner that is more efficient and robustfor transmission and reception of the IBOC signal over the radiochannel. A standard commercially available audio encoder can initiallyproduce the compressed audio frames. An input format converter removesunnecessary information from the audio frames generated by the audioencoder. This unnecessary information includes frame synchronizationinformation as well as any other information, which can be removed ormodified for DAB audio transmission without impairing the audioinformation. An IBOC DAB modem frame assembler reinserts synchronizationinformation in a manner that is more efficient and robust for DABdelivery. A format converter at the receiver repackages the recoveredaudio frames to be decoded by a standard audio decoder.

Both the AM and FM IBOC DAB systems arrange the digital audio and datain units of modem frames. The systems are both simplified and enhancedby assigning a fixed number of audio frames to each modem frame. Ascheduler determines the total number of bits allocated to the audioframes within each modem frame. The audio encoder then encodes the audioframes using the bit allocation for that modem frame. The remaining bitsin the modem frame are consumed by the multiplexed data and overhead.

A functional block diagram of the process for assembling a modem frameis presented in FIG. 2. The functions illustrated in FIG. 2 can beperformed in block 14 of FIG. 1. In this embodiment of the invention,left and right audio DAB programming signals are supplied on lines 50and 52. Data messages (also referred to as auxiliary data) havingvarious levels of priority are supplied on lines 54, 56 and 58, andstored in buffers 60, 62 and 64. A dynamic scheduling algorithm 66, orscheduler, coordinates the assembly of the modem frame with an audioencoder 68. The amount of auxiliary data that may be transmitted isdetermined by multiple factors. In the preferred embodiment, the audioencoder first scans the audio content of the audio information in anaudio frame buffer 70 holding the audio information to be transmitted inthe next modem frame. The scanning is done to estimate the complexity or“entropy” of the audio information for that modem frame, as illustratedby block 72. This entropy estimate can be used to project the targetnumber of bits required to deliver the desired audio quality. Using thisentropy estimate on line 74, along with the quantity and priorityassignments of the data in the messages in buffers 60, 62 and 64, thedynamic scheduling algorithm allocates the bits in the modem framebetween data and audio.

After a number of bits has been allocated for the next modem frame, theaudio encoder encodes all the audio frames (e.g. 64 audio frames) forthe next modem frame and passes its result to the audio frame formatconverter 76. The actual number of bits consumed by the audio frame arepresented to the scheduler on line 78 so it can make best use of theunused bit allocation, if any. The audio frame format converter removesany header information and unnecessary overhead and passes the resulting“stripped” audio frames to the modem frame format and assembly functionblock 80.

The dynamic scheduling algorithm, or scheduler, can generally operate asfollows. First, if no data messages are pending, then the schedulerallocates all the capacity of the next modem frame to the compressedaudio. This would often result in more bits than the target number ofbits required to achieve the desired audio quality. Second, if only lowpriority messages are pending, then the capacity of the modem frame inexcess of the target number of bits for audio is allocated to themessages (data). This should result in no loss of audio quality relativeto that desired. Third, if high priority messages are pending, then thescheduler must make a compromise between the audio quality and thetimely delivery of the high priority messages. This compromise can beevaluated using cost functions assigned to message latency goals versusthe potential reduction in audio quality. The messages to be transmittedcan be selected by sending a signal as illustrated by line 82 to a datapacket multiplexer 84.

From a broadcaster's perspective, higher priority messages areassociated with incremental increases in cost since the audio qualitycan be incrementally affected. From a data or message user perspective,the prioritization of messages can also be based upon a cost function tocompensate the broadcaster for loss of audio quality. This cost functioncan be an actual cost. For example, the actual user cost of packetdelivery can double for each increase in priority class. This can be aneffective means to increase revenue from users willing to pay more thanthe nominal cost if the messages are perceived to be urgent.Alternatively, prioritization can be accomplished by the type of messagegenerated by the broadcaster. In either case the prioritization isself-regulating, and higher priority messages are assigned withdiscretion since there is some incremental cost involved, both to theuser and to the broadcaster. Of course the broadcaster will assign therules and associated cost functions for his net benefit while providinga potentially valuable service to his users and listeners.

The modem frame format and assembly function arranges the audio frameinformation and data packets into a modem frame. Header informationincluding the size and location of the audio frames, which had beenremoved in the audio frame format converter, are reinserted into themodem frame in a redundant, but efficient, manner. This reformattingimproves the robustness of the IBOC DAB signal over theless-than-reliable radio channel. For transmission in the all-digitalIBOC DAB mode, backup frames, based on data supplied on line 86, arealso generated. The backup frames can provide a time diverse redundantsignal to reduce the probability of an outage when the main signalfails. In normal operation, the backup frames are code-combined with themain channel to yield an even more robust transfer of information in thepresence of fading. The analog signal (AM or FM) is used in place of thebackup frames in the Hybrid IBOC system.

The receiver performs the inverse of some of the functions described forthe transmitter. FIG. 3 is a block diagram of a radio receiver 88capable of performing the signal processing in accordance with thisinvention. The DAB signal is received on antenna 90. A bandpasspreselect filter 92 passes the frequency band of interest, including thedesired signal at frequency f_(c), but rejects the image signal atf_(c)−2f_(if) (for a low side lobe injection local oscillator). Lownoise amplifier 94 amplifies the signal. The amplified signal is mixedin mixer 96 with a local oscillator signal f_(lo) supplied on line 98 bya tunable local oscillator 100. This creates sum (f_(c)+f_(lo)) anddifference (f_(c)−f_(lo)) signals on line 102. Intermediate frequencyfilter 104 passes the intermediate frequency signal f_(if) andattenuates frequencies outside of the bandwidth of the modulated signalof interest. An analog-to-digital converter 106 operates using a clocksignal f_(s) to produce digital samples on line 108 at a rate f_(s).Digital down converter 110 frequency shifts, filters and decimates thesignal to produce lower sample rate in-phase and quadrature signals onlines 112 and 114. A digital signal processor based demodulator 116 thenprovides additional signal processing to produce an output signal online 118 for output device 120.

FIG. 4 is a block diagram illustrating the modem frame demodulating ofaudio and data performed in the receiver of FIG. 3. A frame disassembler122 receives the signal to be processed on 124 and performs all thenecessary operations of deinterleaving, code combining, FEC decoding,and error flagging of the audio and data information in each modemframe. The data, if any, is processed in a separate path on line 126from the audio on line 128. The data then is routed as shown in block130 to the appropriate data service. The data priority queuing is afunction of the transmitter, not the receiver. The audio informationfrom each modem frame is processed by a format converter 132 whicharranges the audio information into an audio frame format that iscompatible with the target audio decoder 134 that produces the left andright audio outputs 136 and 138.

In one type of hybrid FM DAB system an analog modulated carrier iscombined with a plurality of orthogonal frequency division multiplexed(OFDM) sub-carriers placed in the region from about 129 kHz to 199 kHzaway from the FM center frequency, both above and below the spectrumoccupied by an analog modulated host FM carrier. In an all-digitalversion, the analog modulated host signal is removed, while retainingthe above sub-carriers and adding additional sub-carriers in the regionsfrom about 100 kHz to 129 kHz above and below the FM center frequency.These additional sub-carriers can transmit a backup signal that can beused to produce an output at the receivers in the event of a loss of themain, or core, signal.

The various frame formats have been carefully constructed to provide anefficient and robust IBOC DAB communications system. Moreover, the frameformatting enables important features of this design; which include timediversity, rapid channel tuning, multi-layer FEC code combining betweenmain and backup channels, redundant header information (a form ofunequal error protection), and flexibility in allocating throughputbetween audio frames and data messages. Many of the features of theframe formats are designed for the all-digital FM IBOC DAB system. TheFM hybrid frame formats are made to be compatible with the FMall-digital formats.

As shown in FIG. 5, the main channel modem frame 140 is comprised of aset of 8 backup core audio (BCAx) fields 142, an optional enhancedaudio/data (EAD) field 144 and a redundant header (RH) field 146. Themain channel modem frame carries audio information for 64 audio frames,along with a dynamic data capacity. In the preferred embodiment, thesize of the modem frame is 18,432 bytes after Reed-Solomon encoding. Thenumber of input bytes for the RS(144,140), RS(144,136) and RS(144,132),coding options are 17,920 bytes, 17,408 bytes, and 16,896 bytes,respectively.

This modem frame is presented to a Reed Solomon encoder and subsequentforward error correction (FEC) and interleaving functions. The rate ofthe Reed Solomon encoder determines exactly how many bytes comprise themodem frame before FEC encoding. It should be noted that in thepreferred embodiment, the Reed Solomon code words are encodedsystematically such that the parity symbols are in front of theinformation symbols. This ensures that the flush byte (all zeroes)remains as the last byte presented to the inner convolutional encoder.The redundant header field is located at the end of the modem frame toensure that it is coded with a separate Reed-Solomon code word.

The format for the backup audio/supplemental frame 148 of theall-digital IBOC DAB system is shown in FIG. 6. Each backupaudio/supplementary frame includes a backup audio field 150, asupplementary data field 152, a cyclic redundancy check byte 154, and aflush byte 156. The two modes of operation include the 24 kbps coreaudio backup mode and the 48 kbps core audio backup. Although each BCAxframe holds 8 audio fields each of variable length, the total length ofthe combined BCAx fields is constant.

The 8 backup core audio fields BCA0 through BCA7 of the main channelmodem frame are redundant with the same fields 142 in the backup/audiosupplemental frame (BAS) 148. However, the backup frames of theall-digital IBOC DAB system are transmitted several seconds after thetransmission of the corresponding modem frame. The backup frames areintentionally delayed for the purpose of introducing the time-diversityfeature. This diversity delay is an integer number of modem frames. Incontrast, the receiver processes the backup frames as quickly aspractical to enable rapid tuning. The receiver time-aligns the BCAxfields in the modem frame with the redundant BCAx fields in the backupframe by appropriately delaying the audio information in the modemframe.

After the BCAx fields in the modem frame and the BCAx fields in thebackup frame have been aligned, the time-aligned BCA fields arecode-combined in the receiver's convolutional decoder. In one embodimentof a transmitter using the signal processing of this invention, an outerReed Solomon FEC is applied to the digital signal, followed by an innerconvolutional FEC, prior to interleaving and subsequent transmission. Itis important that the BCA fields are coded exactly in the same sequencewith both the inner and outer FEC codes to enable the diversity codecombining. This results in robust performance for the tuning and backupchannel, even when both the modem frame and the backupaudio/supplemental frames are partially corrupted. In the preferredembodiment, the BCA fields carry a core backup audio signal at either 24kbps or 48 kbps, selectable by the broadcaster.

The backup audio/supplemental frame BASx is transmitted on the backupchannel sub-carriers during each pair of interleaver blocks over themodem frame duration. The supplementary data field with cyclicredundancy check and flush bytes is transmitted only in the 24 kbps coreaudio backup mode. The supplementary data field is replaced withadditional audio information in the 48 kbps core audio backup mode. Inthe preferred embodiment, the BASx frame includes 1152 bytes (after ReedSolomon encoding), in 8 Reed Solomon codewords. Each BCAx field includes576 bytes (after Reed Solomon encoding) for the 24 kbps mode, in 4 ReedSolomon codewords, or 1152 bytes (after Reed Solomon encoding) for the48 kbps mode, in 8 Reed Solomon codewords. The supplementary data fieldincludes 576 bytes (after Reed Solomon encoding) for the 24 kbps mode,in 4 Reed Solomon codewords. In the 48 kbps mode, the supplementary datafield is not present. The cyclic redundancy check and flush bytes areused in the 24 kbps modes, but not in the 48 kbps mode. The 24 kbpsbackup audio mode enables the insertion of a supplementary data fieldwith a throughput of about 24 kbps. This field is intended for use as anindependent broadcast messaging or data packet delivery service. Theframing at this level simply provides the channel capacity for thesupplementary data, which would have its own formatting/protocol withinthe supplementary data field.

The format for the backup core audio field (BCAX) 142 is presented inFIG. 7. The length of this field is determined by the choice between twobackup modes. A 24 kbps backup mode is intended to provide a monophonicbackup audio signal with an audio bandwidth of about 6 kHz, while audiosignal of a 48 kbps backup mode is stereo or mono with a bandwidth ofabout 10 kHz. The BCAx field holds 8 audio frames 158 each of variablelength, a header field (HCA) 160, a flush byte 162, and possibly a sparefield 164. The spare field includes any bytes remaining after audioframe allocation. Each audio frame includes a core audio frame (CAx) 166and a cyclic redundancy check byte 168. However, the total length of theBCAx field 142 is constant. Therefore, the audio encoder is allotted afixed number of bytes to encode each group of 8 core audio frames (CAx).

One of the backup core audio fields BCAx (x=0 through x=7) isredundantly transmitted on the backup channel sub-carriers over eachinterleaver block (0 through 7) of the modem frame. The 8 BCAx framesare also transmitted as part of the modem frame. In the preferredembodiment, each BCAx field includes 576 bytes (after Reed Solomonencoding) for the 24 kbps mode, in 4 Reed Solomon codewords, and 1152bytes (after Reed Solomon encoding) for the 48 kbps mode, in 8codewords. The core audio frame CAx holds variable length audio framenumber of bytes (before Reed Solomon encoding) in CAx fields indicatedin the header CAx fields ordered for improved error concealment. A onebyte (before Reed Solomon encoding) cyclic redundancy check is included,as is a one byte (before Reed Solomon encoding) flush field to flush theViterbi decoder. The HCA header is 8 bytes (before Reed Solomonencoding), and indicates the size of the each of the 8 CAx fields.

The enhanced audio/data (EAD) 170 field format is presented in FIG. 8.The EAD is transmitted within the modem frame and holds audioenhancement information for 64 audio frames. The EAD includes a headerfield 172, a plurality of enhanced audio fields 174, each including anenhanced audio portion (EAx) 176 and a cyclic redundancy check byte 178,a data field 180, another cyclic redundancy check byte 182 and a flushbyte 184. The preferred embodiment of the EAD contains 13680 bytes(after RS encoding) for 24 kbps B/U mode, with 95 RS codewords, and 9072bytes (after RS encoding) for 48 kbps B/U mode, with 63 codewords. A 64byte (before RS encoding) header 166 indicates the size of each of 64EAx fields 168. The EAx fields hold audio enhancement information toincrease the core quality/rate. The number of bytes (before RS encoding)in each EAx field, is indicated in the header, x=0, 7, 14, . . . (7*kmod 64), for k=0 to 63, ordered for error concealment. Each enhancedaudio field includes a data portion 170, and a cyclic redundancy checkbyte 172. If the scheduler determines that bytes are available for data,the data can be carried in data field 174, with a cyclic redundancycheck byte 178. A one byte (before RS encoding) zero flush field 178 isused to flush the Viterbi decoder. The EAD field carries the additionalaudio information such that, when combined with the core audio fields ofthe corresponding modem frame, provides virtual compact disk (CD)quality sound.

The enhanced audio/data field includes a header field 172, a pluralityof enhanced Audio Fields 174, each including an audio portion (EAx) 176and a cyclic redundancy check byte 178, a data field 180, another cyclicredundancy check byte 182, and a flush byte 184. The redundant header(RH) field format 146 is presented in FIG. 9. This field carriesredundant information regarding the sizes (or locations) of the audiofields. It includes redundant header field (HEA) 172, core audio headers(HCAx) 186, a cyclic redundancy check byte 188, and a flush byte 190.The redundant header field carries header information for the 64 audioframes within the modem frame. In the preferred embodiment, theredundant header field includes 144 bytes (after Reed Solomon encoding),in one codeword. The HEA includes 64 bytes (before Reed Solomonencoding) indicating the size of each of the 64 EAx fields, and isredundant with the HEA field in the EAD frame. The core audio headerincludes 64 bytes (before Reed Solomon encoding) in 8 headers duplicatedfrom BCA's. A single byte cyclic redundancy check is included over allheaders. The flush field includes 15-P zero bytes (before Reed Solomonencoding), where P is the number of parity bytes, to flush the Viterbidecoder. This redundancy provides additional protection againstcorruption of the important header information. The enhanced audioheaders (HEA) 166 are transmitted in two locations within the modemframe (i.e., the RH field and the 8 EAD field). The core audio headers182 are transmitted in three locations (i.e., the RH and the 8 HCAfields within the modem frame, in addition to the 8 HCA fields in thebackup audio supplemental (BAS) frames of the all-digital IBOC DABsystem). The HEA header information includes 64 bytes (before RSencoding) indicating the size of each of the 64 EAx fields redundantwith the HEA field in the EAD frame. The core audio headers include 64bytes (before RS encoding), with eight headers duplicated from the BCAs.The RH field includes 144 bytes after RS encoding, with one RS codeword.The RH Field also includes a cyclic redundancy check byte 184 and aflush field 186. The number of bytes of the flush field is a function ofthe number of parity bytes (P) in the Reed-Solomon coding. Specificallythe number of flush bytes equals 15-P.

In an embodiment of the invention particularly applicable to AM DABsystems, the data is segregated into Core Data or Enhancement Data,depending upon the desired coverage requirements. The AM DAB Modem Frame192 illustrated in FIG. 10 includes a set of 8 Backup Core Audio fields194, an Enhanced Audio/Data field 196 and a Redundant Header field 198,as shown in the diagram of FIG. 10. Each Backup Core Audio fieldincludes a group of 4 Core Audio Frames, where each BCA field isallocated a fixed maximum size. The composite Modem Frame is presentedto the CPTCM Encoder and subsequent interleaving functions.

The format for the Core Audio Block 194 of the Core Modem is presentedin FIG. 11. Each CAB includes a header 200, four Core Audio frames 202,each with a cyclic redundancy check byte 204, a spare block 206, and aflush field 208. The eight CABx frames are transmitted as part of thecore modem frame. In the preferred embodiment, each CABx field is 460bytes before coding. The HCA header is four bytes, indicating the sizeof each of the four CAx fields. The core audio frame CAx holds avariable length audio frame number of bytes in CAx indicated in theheader. CRC is a 1-byte cyclic redundancy check. Block 206 representsspare bytes remaining (if any) after audio frame allocation. The flushblock 208 is six bits of zero data used to flush the Viterbi decoder.

The Audio Encoder of FIG. 3 is allocated a number of bits for the nextModem Frame (Core or Enhancement). The Audio Encoder encodes all theAudio Frames (e.g. 32 Audio Frames) for the next Modem Frame and passesits result to the Audio Frame Format Converter.

The AM DAB Core Modem format carries core audio information for 32 audioframes, along with a dynamic data capacity. The Core Modem Frame iscomprised of time-diverse main and backup components. In the preferredembodiment, the size of the Core Modem Frame is 30,000 bits (3750 bytes)before coding. CABx (x=0 to x=7) represent the core audio blocks CSB0through CSB7 of 460 bytes each.

The eight Core Audio fields CAB0 through CAB7 of the Modem Frame aretransmitted redundantly as time diverse Main and Backup components.These Main and Backup components are created in the FEC coding andinterleaving process. The Backup component of the All-Digital IBOCsystem are transmitted several seconds after the transmission of thecorresponding Main component of the Core Modem Frame. The Backupcomponent is intentionally delayed for the purpose of introducing thetime-diversity feature. This diversity delay is an integer number ofCore Modem Frames (e.g. 3). In contrast, the receiver processes theBackup component as quickly as practical to enable rapid tuning. Thereceiver deinterleaves the Backup and Main components of the Core ModemFrame such that these components, when available, are code-combinedafter taking advantage of the diversity gain and metric estimation.

The Enhancement Modem Frame (EMF) 210 format is presented in FIG. 12.Each EMF frame includes a header 212, a plurality of Enhanced Audiofields (EAx), each having a cyclic redundancy byte 216, a spare block218, and a flush field 220. This frame carries the additional audioinformation such that, when combined with the Core Audio of thecorresponding Core Modem Frame, provides higher audio quality than theCore alone.

The enhancement mode frame holds the audio enhancement information for32 audio frames, plus data, if any. In the preferred embodiment, theenhancement modem frame holds 22,800 bits (3360 bytes). The HEA 212header contains 32 bytes, indicating the size of each of the 32 EAxfields. The EAx fields hold enhancement audio information to increasethe core audio quality, and are of variable size. A one bit cyclicredundancy check is provided. Block 218 contains any spare bytesremaining after audio frame allocation. A one byte flush field of zerosis included to flush the Viterbi decoder.

The scheduler orders the incoming prioritized and packetized messagesbased upon some predefined rules. The simplest algorithm would simplyplace the highest priority message packets in the front of the queue inchronological order for each priority class. This algorithm wouldguarantee that higher priority messages would be transmitted before anylower priority messages waiting in the queue, and the chronologicalorder would ensure fairness within each priority class. It also ensuresthat the highest priority message class will be transmitted with theshortest possible delay of any conceivable scheduling algorithm.However, this particular scheduling algorithm does not ensure thatmessages would be delivered within guaranteed times for each priorityclass. Moreover, it is possible for a message of any priority other thanthe highest to be in the queue indefinitely as new highest prioritymessages continue to be generated.

The various frame formats have been carefully constructed to provide anefficient and robust AM IBOC DAB communications system. Moreover, theframe formatting enables important features of this design, whichinclude time diversity, rapid channel tuning, multi-layer FEC codecombining between main and backup channels, and flexibility inallocating throughput between audio frames and data messages. Many ofthe features of the frame formats are designed for the All-Digital AMIBOC DAB system. The AM Hybrid Frame formats are made to be compatiblewith the AM All-Digital formats.

FIG. 13 is a block diagram of the advanced audio coding (AAC) IBOC DABinterfaces in a receiver constructed in accordance with this invention.The incoming signal is provided from the receiver air interface on line222. A modem and frame disassembler 224 separates the data from theencoded frame boundary information and the audio information. The dataare sent on line 226 to a data router 228 that sends the data to variousdestinations on line 230. The boundary and audio information aresupplied on lines 232 and 234 to a format converter 236 that convertsthe signal into a standard AAC bit stream on line 238. Then a standardAAC decoder 240 decodes the audio samples.

FIG. 14 is a block diagram of an AAC/IBOC DAB interface in a transmitterconstructed in accordance with this invention. A modem frame audiostream is supplied on line 242 to an AAC encoder 244. The AAC encoderinitially produces an entropy signal on line 246 for modem frame dataallocater 248. A data scheduler 250 supplies data at various prioritiesto the modem frame data allocater on lines 252. The modem frame dataallocater 248, produces a bit allocation signal on line 254. Then theAAC encoder produces an AAC audio bit stream on line 256. Formatconverter 258 converts the standard AAC bit stream to encoded frameboundary information on line 260, and encoded frame audio information online 262. An allocation variance signal is also provided on line 264,permitting the modem frame data allocater to allocate data on line 266in accordance with the allocation variance signal. The modem frameassembler 268 receives the encoded frame boundary information, theencoded frame audio information, and the data allocated in accordancewith the allocation variance signal to produce the modem frame that isoutput to the air interface on line 270.

The scheduler orders the incoming prioritized and packetized messagesbased upon some predefined rules. The simplest algorithm would simplyplace the highest priority message packets in the front of the queue inchronological order for each priority class. This algorithm wouldguarantee that higher priority messages would be transmitted before anylower priority messages waiting in the queue, and the chronologicalorder would ensure fairness within each priority class. It also ensuresthat the highest priority message class will be transmitted with theshortest possible delay of any conceivable scheduling algorithm.However, this particular scheduling algorithm does not ensure thatmessages would be delivered within guaranteed times for each priorityclass. Moreover, it is possible for a message of any priority other thanthe highest to be in the queue indefinitely as new highest prioritymessages continue to be generated.

More complicated dynamic scheduling algorithms could be employed thatguarantee delivery times for each priority class. A flow controlmechanism may also prevent the acceptance of the message in the queue ofa priority class when it is full. At least the user knows whether or notthe delivery time is guaranteed. If a particular priority class is full,the user could schedule his message in another priority class with adifferent cost. One advantage of this algorithm is the mechanism thatprevents hang-up of lower priority messages when the higher prioritymessages are constantly being generated. In addition, the user pays onlyfor the service he receives. To summarize, there is considerableflexibility is choosing a scheduling algorithm with associated costfunctions to enable the broadcaster to optimize his services.

This invention provides a robust method for the multiplexing andtransmission of compressed digital audio frames along with digital datapackets within a modem frame in In-Band On-Channel (IBOC) Digital AudioBroadcasting (DAB) systems. This method is designed to have minimumadverse impact on the digital audio quality while maximizing datathroughput for multiple messages with different priority assignments.The invention provides a flow control mechanism where a compromise isoptimized, given assigned priorities of classes of message packetsversus audio quality. A scheduling algorithm for the various packetpriorities multiplexes the data packets along with the encoded audiopackets during assembly of the modem frame. Additionally, audio frameformat converters are used to enable transmission of reformatted genericcompressed audio frames in the DAB modem frame in a manner that istransparent to the audio decoder. However some restrictions are placedon the audio encoder. These encoder restrictions are related to theallotment of bits to various groupings of audio frames. The new frameformatting enables time diversity transmission of audio information aswell as FEC code combining of the time-diverse audio segments in anall-digital system. This time diversity feature and its compatibilityare also maintained in the hybrid system, which uses the analog signalas a time-diverse backup, as shown in U.S. patent application Ser. No.08/947,902, filed Oct. 9, 1997, assigned to the assignee of thisinvention, now U.S. Pat. No. 6,178,317.

The present invention permits the use of a standard advanced audiocoding (AAC) encoder in a digital audio broadcasting transmitter. In theillustrated preferred embodiment of the transmitter, the custom modemframe formatting is performed outside of the encoder. Similarly, thepreferred embodiment of the receiver disassembles the modem frame priorto using a standard AAC decoder to decode the audio samples.

While the present invention has been described in terms of its preferredembodiment, it will be understood by those skilled in the art thatvarious modifications can be made to the disclosed embodiment withoutdeparting from the scope of the invention as set forth in the claims.

What is claimed is:
 1. A method for transmission of compressed data fora digital audio broadcasting system comprising the steps of: receivingdigital information representative of an audio signal; estimating anumber of bits to be allocated to said digital information in a modemframe; encoding said digital information within the estimated number ofbits to produce encoded data; adding bits corresponding to digitalmessages to said encoded information to form a composite modem frame;formatting said composite modem frame bits to produce formattedcomposite modem frame bits; and transmitting the formatted compositemodem frame bits.
 2. The method of claim 1, wherein the step ofestimating the number of bits to be allocated to encode said digitalinformation in a modem frame, comprises the steps of: storing saiddigital information in a buffer; and estimating the entropy of saiddigital information.
 3. The method of claim 1, further comprising thestep of: removing selected overhead bits from said encoded data.
 4. Themethod of claim 1, wherein the step of adding bits corresponding todigital messages to said encoded information to form a composite modemframe, comprises the steps of: prioritizing a plurality of said digitalmessages; and selecting bits of said digital messages having the highestpriority to be added to available bits in said modem frame.
 5. Themethod of claim 1, wherein the step of formatting said composite modemframe bits to produce formatted composite modem frame bits, comprisesthe step of: inserting redundant frame overhead data into said compositemodem frame.
 6. The method of claim 1, further comprising the step of:multiplexing said digital messages and inserting the multiplexed digitalmessages into said composite frame data.
 7. The method of claim 1,wherein said modem frame includes a fixed number of audio frames, saidaudio frames having variable lengths.
 8. The method of claim 1, whereinthe step of encoding said digital information within the estimatednumber of bits to produce encoded data comprises the step of: arrangingthe bits of digital information into a plurality of backup frames and anenhanced audio frame.
 9. The method of claim 8, wherein the bits ofdigital information in said backup frames and said enhanced audio frameare arranged to be subsequently code combined.
 10. A transmitter for adigital audio broadcasting system comprising: means for receivingdigital information representative of an audio signal; means forestimating the number of bits to be allocated to said digitalinformation in a modem frame; means for encoding said digitalinformation within the estimated number of bits to produce encoded data;means for adding bits corresponding to digital messages to said encodedinformation to form a composite modem frame; means for formatting saidcomposite modem frame bits to produce formatted composite modem framebits; and means for transmitting the formatted composite modem framebits.
 11. The transmitter of claim 10, wherein the means for estimatingthe number of bits to be allocated to encode said digital information ina modem frame, comprises: means for storing said digital information ina buffer; and means for estimating the entropy of said digitalinformation.
 12. The transmitter of claim 10, further comprising: meansfor removing selected bits from said encoded data.
 13. The transmitterof claim 10, wherein the means for adding bits corresponding to digitalmessages to said encoded information to form a composite modem frame,comprises: means for prioritizing a plurality of said digital messages;and means for selecting bits of said digital messages having the highestpriority to be added to available bits in said modem frame.
 14. Thetransmitter of claim 10, wherein the means for formatting said compositemodem frame bits to produce formatted composite modem frame bits,comprises: means for inserting redundant frame overhead data into saidcomposite modem frame.
 15. The transmitter of claim 10, furthercomprising: means for multiplexing said digital messages and insertingthe multiplexed digital messages into said composite frame data.
 16. Thetransmitter of claim 10, wherein said modem frame includes a fixednumber of audio frames, said audio frames having variable lengths. 17.The transmitter of claim 10, wherein the means for encoding said digitalinformation within the estimated number of bits to produce encoded datacomprises: means for arranging backup frames of said digital informationfor transmission within said composite modem frame.
 18. The transmitterof claim 17, wherein the bits of digital information in said backupframes and said enhanced audio frame are arranged to be subsequentlycode combined.
 19. A transmitter for a digital audio broadcasting systemcomprising: an input for receiving digital information representative ofan audio signal; a processor for estimating the number of bits to beallocated to said digital information in a modem frame, for encodingsaid digital information within the estimated number of bits to produceencoded data, for adding bits corresponding to digital messages to saidencoded information to form a composite modem frame, and for formattingsaid composite modem frame bits to produce formatted composite modemframe bits; and an antenna for transmitting the formatted compositemodem frame bits.
 20. The transmitter of claim 19, wherein the processorestimates the entropy of said digital information.
 21. The transmitterof claim 19, wherein the processor removes selected bits from saidencoded data.
 22. The transmitter of claim 19, wherein the processorprioritizes a plurality of said digital messages and selects bits ofsaid digital messages having the highest priority to be added toavailable bits in said modem frame.
 23. The transmitter of claim 19,wherein the processor inserts redundant frame overhead data into saidcomposite modem frame.
 24. The transmitter of claim 19, furthercomprising: a multiplexer for multiplexing said digital messages andinserting the multiplexed digital messages into said composite framedata.
 25. The transmitter of claim 19, wherein said modem frame includesa fixed number of audio frames, said audio frames having variablelengths.
 26. The transmitter of claim 19, wherein the processor arrangesbackup frames of said digital information for transmission within saidcomposite modem frame.
 27. The transmitter of claim 26, wherein the bitsof digital information in said backup frames and said enhanced audioframe are arranged to be subsequently code combined.
 28. The transmitterof claim 19, wherein the processor is a programmable digital signalprocessor.
 29. The method of claim 1, wherein the steps of estimating anumber of bits to be allocated to said digital information in a modemframe, encoding said digital information within the estimated number ofbits to produce encoded data, and receiving digital messages, addingbits corresponding to digital messages to said encoded information toform a composite modem frame, and formatting said composite modem framebits to produce formatted composite modem frame bits, are performed by adigital signal processor.
 30. The method of claim 29, wherein thedigital signal processor is a software programmable digital signalprocessor.
 31. A method for transmitting and receiving compressed datafor a digital audio broadcasting system comprising the steps of:receiving digital information representative of an audio signal;estimating a number of bits to be allocated to said digital informationin a modem frame; encoding said digital information within the estimatednumber of bits to produce encoded data, and receiving digital messages;adding bits corresponding to digital messages to said encodedinformation to form a composite modem frame; formatting said compositemodem frame bits to produce formatted composite modem frame bits;transmitting the formatted composite modem frame bits; receiving themodem frame bits; and producing an output in response to the modem framebits.
 32. The method of claim 31, wherein the step of estimating thenumber of bits to be allocated to encode said digital information in amodem frame, comprises the step of: estimating the entropy of saiddigital information.
 33. The method of claim 31, further comprising thestep of: removing selected overhead bits from said encoded data.
 34. Themethod of claim 31, wherein the step of adding bits corresponding todigital messages to said encoded information to form a composite modemframe, comprises the steps of: prioritizing a plurality of said digitalmessages; and selecting bits of said digital messages having the highestpriority to be added to available bits in said modem frame.
 35. Themethod of claim 31, wherein the step of formatting said composite modemframe bits to produce formatted composite modem frame bits, comprisesthe step of: inserting redundant frame overhead data into said compositemodem frame.
 36. The method of claim 31, further comprising the step of:multiplexing said digital messages and inserting the multiplexed digitalmessages into said composite frame data.
 37. The method of claim 31,wherein said modem frame includes a fixed number of audio frames, saidaudio frames having variable lengths.
 38. The method of claim 31,wherein the step of encoding said digital information within theestimated number of bits to produce encoded data comprises the step of:arranging the bits of digital information into a plurality of backupframes and an enhanced audio frame.
 39. The method of claim 38, whereinthe bits of digital information in said backup frames and said enhancedaudio frame are arranged to be subsequently code combined.